Pop your popcorn and place into large greased pan (or 2 9x13 cake pans if you don't have a huge roaster pan)
In a large saucepan, heat brown sugar, butter, and Karo syrup. Bring to a boil and boil for 5 minutes.
Remove from heat and stir in baking soda and vanilla. The caramel will get "fizzy" looking. Don't worry, that's what you want.
Pour the caramel evenly over the popcorn (if using 2 pans, divide between the two pans)
Toss to coat.
Bake at 350 degrees for 15-20 minutes.
While baking, stir the caramel very 5 minutes to ensure it does not burn. You know it's done when the popcorn looks "dry" When you pull it out of the oven, sprinkle generously with flaky sea salt.
Turn out of the pan and break apart. Allow to cool and enjoy!
